1. Field of the Invention
The present invention relates to a magnetron for a microwave oven, and more particularly to a magnetron for a microwave oven wherein a spacer having a disc shape is fitted between a lower pole piece and an F-seal supporting the lower pole piece and provided at its upper surface with a metal coating, thereby removing effectively leaked microwaves and avoiding a lateral vibration of a lead assembly.
2. Description of the Prior Art
Generally, a magnetron for a microwave oven comprises a diode for emitting thermions. Referring to FIG. 1, there is illustrated an example of a conventional magnetron for a microwave oven. As shown in FIG. 1, the magnetron comprises a magnetron body 1, and a filament 2 disposed in the magnetron body 1 and adapted to emit thermions. The magnetron body 1 is disposed in a casing constituted by an upper member 5 of a plate shape and a lower member 6 of a cylindrical shape. The magnetron body 1 also has upper and lower portions protruded beyond upper and lower members 5 and 6, respectively. To seal and support the magnetron body 1, an A-seal member 3 and an F-seal member 4 are provided at the upper and lower portions of the magnetron body 1. The seal members 3 and 4 also function as a magnetic path. Around the filament 2, a vane 7 is placed to receive microwave energy generated when the thermions emitted from the filament 2 are acceleratively rotated in an interaction spacer 9. A strap 16 is also provided for adjusting frequencies of the thermions rotating acceleratively in the interaction space 9. The magnetron also comprises an antenna feeder 8 adapted as a microwave transmission path for guiding microwave energy received by the vane 7 into a cooking chamber. In the casing, upper and lower permanent magnets 10 and 10' are attached to upper and lower members 5 and 6, respectively, to generate a magnetic field. The magnetic field is applied to the interaction space 9, by means of pole pieces 11 and 11'. Around the magnetron body 1, a plurality of cooling fins 12 are disposed which function to release outwardly heat generated at the side of vane 7 and thus cool the interior of magnetron body 1. A center lead 17 and a side lead 18 are connected at their one ends to both ends of the filament 2, respectively, so as to apply electric power to the filament 2. To the other ends of leads 17 and 18, a through type condenser 14 is connected, which functions as a terminal making it possible to apply easily electric power from the outside to the filament 2. A choke coil 15 is also provided to remove conductive noise generated by lead current. The condenser 14 cooperates with the choke coil 15 to enhance a shield effect on conductive noise. Beneath the housing, a filter box 13 is disposed to surround the lower portion of the magnetron body 1. The filter box 13 functions to remove radiation noise emitting through both the center lead 17 and the side lead 18. A spacer 19 is also provided to support both the center lead 17 and the side lead 18. To the lower end of the magnetron body 1, a cathode terminal 20 is mounted. An F-ceramic member 21 is also provided between the F-seal member 4 and the cathode terminal 20.
In this conventional magnetron with the above-mentioned construction, as electric power is applied to the filament 2 via the center lead 17 and the side lead 18, the filament 2 emits thermions which are, in turn, radiated into the interaction space 9. In the interaction space 9, the thermions conduct a cycloidal movement, that is, an accelerated rotation, by axial magnetic fluxes generated from the pole pieces 11 and 11' and an electric field generated between the filament 2 and the vane 7. On the other hand, microwave energy transmitted to the vane 7 is fed into the cooking chamber, via the antenna feeder 8 and a waveguide (not shown) of the oven, thereby heating the food placed in the cooking chamber.
At this time, the magnetron generates microwaves which includes basic frequency of 2.45 GHz and harmful higher harmonics having a frequency corresponding to a multiple of the basic frequency.
Although such microwaves are desired to go to the output part of magnetron, namely, the antenna feeder 8, in actual, a part of the microwaves flows usually toward the inlet part of magnetron, via the center lead 17, the side lead 18 and the cathode terminal 20.
Such a flow of microwaves into the input part of magnetron results in a degradation in efficiency of the magnetron. Furthermore, if excessive microwaves pass through the magnetron, overheating of the magnetron occurs and results in a damage of the choke coil 15 which is of a structure adapted to attenuate the microwaves in its path. Upon being outwardly leaked, this excessive microwaves also may exert a harmful influence on human bodies and cause radio interference for other appliances such as televisions and etc.
In order to avoid such a leakage of microwaves, there has been proposed microwave shielding devices. A typical example of such microwave shielding devices is illustrated in FIG. 2. As shown in the drawing, the microwave shielding device comprises a microwave shielding choke 22 having a certain shape and fixed to the inner wall of F-seal 4.
In this conventional microwave shielding device, the leakage of microwave is effectively prevented by the microwave shielding choke 22. However, the device requires use of a separate jig for fixing the choke 22, which causes a deterioration in workability in the manufacture of magnetrons and an expensive manufacture cost.
On the other hand, since thermions conduct an accelerated rotation in the interaction space 9, a mechanical vibration occurs at the cathode part of magnetron including the filament 2, the center lead 17 and the side lead 18. U.S. Pat. No. 4,684,845 discloses a device for preventing both the center lead and the side lead from vibrating due to such a mechanical vibration and for maintaining a proper space between the center lead and the side lead. In case of the patent, a spacer is mounted to upper portions of the leads.
The spacer serves effectively to hold the leads at their spaced state. If both the leads vibrate laterally at the same time, however, the spacer then vibrates laterally. As a result, a vibration restraining effect is reduced. Moreover, it is required to form a groove for positioning the spacer at a curved portion of the center lead. It is also needed to provide sleeves. These requirements make a deterioration in workability in the manufacture of magnetrons and an increase in manufacture cost.